Check this thread
out. I know he's built a couple plywood tanks and has "perfected" his technique. You should also check out mfk, bunch of plywood tanks there.
I think the weak spot will be where the 2 glass panels meet up. I'm assuming you are going to have 2 "viewing panels" for the front of the tank. As long as you brace it, you should be fine. I've only built one plywood tank out of the scraps of a 55 gal. It was only ~4ft wide so I'm not sure how strong that midpoint would be.